The present invention relates to a spark plug for igniting fuel in an internal combustion engine.
Conventionally, spark plugs have been used for igniting fuel in internal combustion engines of automobiles and the like. In a typical spark plug, an insulator having an axial bore holds a center electrode in a front end portion of the axial bore, and an electrical terminal in a rear end portion of the axial bore. A metallic shell holds the insulator therein while surrounding a trunk portion thereof. One end of a ground electrode is welded to a front end surface of the metallic shell and the other end of the ground electrode is bent so as to face the center electrode, thereby forming a spark discharge gap therebetween. A spark discharge is induced across the spark discharge gap.
An insulator of such a spark plug is manufactured in the following manner. First, a material powder which predominantly contains electrically insulative ceramic, such as alumina, is rubber-pressed into a green compact having a preliminary shape of the insulator. Since a press pin is set in a rubber mold for rubber pressing, a through-hole is formed in the green compact. The through-hole becomes the axial bore of the insulator. Next, a support pin is inserted into the through-hole of the green compact from the proximal end of the green compact. The support pin is fixed at its proximal end on a manufacturing apparatus. The green compact is rotatably supported by the support pin. A grindstone is caused to abut the green compact from a direction perpendicular to the axis of the insulator. The grindstone grinds the outer surface of the green compact, thereby forming a preform having the profile of the insulator. Subsequently, the preform undergoes firing, marking, glazing, glost firing, and the like, whereby the insulator is completed (refer to, for example, Japanese Laid-Open Patent Application (kokai) No. 2001-176637).
In recent years, automobile engines have provided increasingly high output with reduced fuel consumption. Under such circumstances, in order to ensure the necessary degree of freedom in designing engines, a reduction in the size of spark plugs has been demanded. In order to reduce the size of a spark plug, the diameter of a metallic shell must be reduced which, of course, requires reducing the diameter of the insulator held in the metallic shell. Such size reductions potentially involve a failure to impart sufficient strength and insulating properties to the insulator. In order to avoid this problem, the diameter of the axial bore of the insulator may be reduced so as to increase the wall thickness of the insulator (the distance between the outer circumferential surface of the insulator and the wall surface of the axial bore). This is accompanied by a reduction in the diameter of the support pin which is used in the process of manufacturing the insulator.
However, in a step of grinding a green compact in the process of manufacturing the insulator, using a support pin whose diameter is smaller than that used in conventional practice raises a problem that the support pin is deflected by the stress induced by contact between the green compact and the grindstone. Particularly, when an insulator having an overall (axial) length of 65 mm or more is to be manufactured, a support pin must be elongated accordingly. As compared with the case of manufacturing an insulator having a short overall length, the barycenter of the insulator is biased more toward a distal end of the insulator. As a result, stress tends to concentrate on a proximal end portion of the support pin to be fixed on the manufacturing apparatus. Grinding a green compact with the support pin being deflected causes a large positional deviation (a large degree of eccentricity or run out) of the center of the through-hole from the center of the outer circumference, particularly at the distal end of the preform. Thus, the wall thickness of the preform is uneven. If an insulator from the preform is attached to a metallic shell, the distance between the outer surface of a thick-walled portion of the insulator and the inner circumferential surface of the metallic shell becomes short, potentially resulting in occurrence of lateral sparks therebetween.